The present invention refers to a process for the production of an alkaline solution of polyhydroxybenzyls, polyhydroxyarylethers, its copolymers or mixtures, which is used to coat the internal surface of vinylic monomer polymerization reactors and has the effect of preventing the accumulation of the polymer on the walls of the reaction vessel.
In the industrial production of vinyl polymers a great variety of chemical procedures are used, among which are particularly relevant to our invention those via suspension, dispersion, microsuspension and emulsion. In those procedures, the reaction medium consists mainly of: water, suspension agents, dispersants or emulsifiers (depending on the case) and one or several substituted monomers of ethylene (typically insoluble in water).
Depending on how stable that heterogeneous and discontinuous medium is, vigorous shaking or other kinds of devices might be needed to keep separated the small entities rich in monomer), where the chemical transformation is carried out. Given the unstabilities associated with the heterogeneous medium or with the imperfections in the homogenizing flux pattern, some polymer particles may tend to separate from the bulk of the solution and to adhere to the inner surfaces of the reactor. As this polymer remains adhered during several reaction cycles, it can cross-link (form a polymer of larger molecular weight than the initial one), and serve as a seed for gradual accumulation of more material.
The adherence of polymeric material to the walls of a reaction vessel as well as to the stirring accessories (rods, flux-breakers, etc) can reduce the quality of the polymer being prepared. The material adhered to those surfaces affects the process in several ways: it reduces thermal conduction between the heat transfer system and the reaction medium, it can affect the homogenizing performance of the mixing elements, by modifying their effective shapes and sizes, and it alters the optical and mechanical properties of the final product when it peels off the walls and passes to the polymer bulk, since such granules are substantially harder and less porous than the normal polymer. Besides, the accumulation of scales must be reduced to a maximum since: 1) its removal implies an extra downtime and consequently a decelaration of production, 2) the opening of the reactor mouth for its proper cleaning (mechanical and/or with pressurized water), allows the release of the monomer to the plant environment, which hinders the maintenance of monomer concentration within the stipulated limits for environmental protection and industrial safety regulations, and contributes to lower the efficiency of the process through the waste of reactant.
Several products have been patented to be used as coatings for polymerization reactors in order to prevent the accumulation of polymer on their surfaces, among which we can mention: the U.S. Pat. No. 4,256,864, which consists of the application of an aqueous solution of a complex, this complex results from mixing a hydrophilic uncross-linked polymer (as polyvinyl alcohol or hydroxypropyl methyl cellulose) with an inhibitor such as a disodium salt of bisfenol A or glucose thiosemicarbazone; in this case the use of an inhibitor brings as a consequence the necessity of rinsing carefully the excess of solution so that it does not affect the polymerization reaction in an undesirable way; the U.S. Pat. No. 4,579,758, in which the base substance for the coating is an oligomer, obtained from the condensation of benzaldehyde and resorcinol, here, the apparently limited commercial availability of the oligomer suggests its production in plant; the U.S. Pat. No. 4,622,245, which utilizes as surface coating the blend of a polymeric compound which contains hydroxyl groups (such as polyvinyl alcohol) and a derivative of salicylic acid, however its use is conditioned to preheating the surfaces of the reactor at temperatures between 40.degree. to 100.degree. C., rinsing carefully after the coating application; and the Canadian Patent CA-1,181,899 (equivalent to the Japanese original JP-54-107991), which deals with the use of phenol-formaldehyde resins, and which is classified by the patent literature as of moderate performance (U.S. Pat. No. 4,431,783).
This short summary of the state of the art shows the extensive variety of compounds and methods used in the area of polymerization reactor coating. However, new materials and procedures are under study aiming at effectively diminishing the formation of scales.